One object of networks designed according to the Worldwide Interoperability for Microwave Access (WiMAX) standard is to improve a spectral efficiency of networks, while keeping the cost of deployments to a minimum. Fixed WiMAX is based on the IEEE 802.16d standard, and mobile WiMAX on the IEEE 802.16e standard.
One way to do this is to use analog beam forming (ABF). The basic principle of ABF is to form a beam at a base station (BS) for each sector in the cell where the BS is located. This can be achieved with a linear antenna array concatenated with a Butler matrix, see IEEE C802.16m-08/182r3.
The BS switches through the available beams, both in the uplink (UL) and the downlink (DL), in an arbitrary sequence, and communicates, at each instant in time, with the mobile stations (MSs) located in the sectors at the respective beams are directed. Due to the beamforming gain, the range of the cell is extended, which is important, especially for BSs that are sparsely deployed, e.g., in rural areas.
ABF is not the theoretic optimal way of using multiple antenna elements. Heterodyning all the signals to and from the baseband, and digitally processing the signals can achieve a higher capacity; see U.S. Pat. No. 6,307,506, “Method and apparatus for enhancing the directional transmission and reception of information.” However, ABF presents an excellent tradeoff between performance and complexity. For example, ABF can be performed with only a single radio frequency (RF) chain.
As another advantage, ABF can be combined with spatial multiplexing, and other MIMO techniques. The set of N available antennas can be partitioned into K groups of M antennas, i.e., M×K=N, so that K beams are formed. In each beam, M antenna elements are available for spatial multiplexing. When dual-polarized antennas are used, it is easily possible to use K=4, and N=2.
Interference Reduction with ABF
ABF can also be used to reduce the interference. MSs receiving different beams in the various sectors are served at different times. Therefore, if the BSs in two adjacent cells arrange the downlink and uplink transmission in such a way that the BSs do not transmit to the MSs in same sector at the same time, the interference at the MSs is greatly reduced.
If the BSs in adjacent cells can coordinate the beams, then interference from the BS in adjacent cells can be substantially reduced. If the BSs cannot coordinate, then the sequence in which beams are transmitted can be selected randomly and independently at each BS. This still leads to a stochastic reduction of the interference, similar to the reduction of interference in random frequency hopping or time-hopping impulse radio.
If ABF is to be used, the BS broadcasts that it is using sequential beamswitching, so that the MSs can take this into account for making their handover decisions. The specific switching sequence can be determined at each BS based using the base station identification (BS ID) as an initial value, i.e., a seed, to a shift register that generates the random switching sequence.
In the related Patent Application, a superframe is partitioned into multiple sequential zones, one zone for each beam. Each zone begins with a preamble. The MS selects the beam for which the signal during the preamble has a largest signal strength, and feeds back the corresponding beam index to the BS. This means the MS needs to know the number of beams the BS is using. Thus, the beamforming is not transparent to the MS. Essentially, there the grouping and beam selection is performed by the MS. It is desired to make the entire beamforming and grouping process transparent to the MS.